Patch age since disturbance drives patch dynamics for flycatchers breeding in both reservoir and riverine habitat

Abstract

Species dependent upon early-successional landscapes often occupy patches at different stages of recovery after disturbance. The demographic processes that drive patch dynamics in these systems have rarely been described but are important for developing effective conservation and management plans, especially when humans have modified the timing and intensity of disturbances that drive regeneration. In riparian systems, disturbance by floods historically initiated plant regeneration, but many rivers are now regulated and stream flows disrupted by dams and reservoirs. We studied the demography and patch dynamics of an endangered, neotropical migrant bird dependent on remnant riparian patches for breeding, the southwestern willow flycatcher (Empidonax trailli extimus), over 9 yr at both a riverine and reservoir site in central Arizona. We found that at both sites, number of territories/ha within patches increased for 2–4 yr after colonization and then declined, with several patches abandoned after 6–10 yr. Age of birds increased with patch age, with younger birds in colonizing patches and older, site-faithful birds in older patches, while mean per capita reproductive success did not differ with patch age. Natal dispersal and breeding dispersal were primarily from intermediate-aged patches into either young- or other intermediate-aged patches. At both riverine and reservoir sites, both the number of patches and the number of territorial birds increased over time, with the percentage of territories shifting into younger and younger patches. The type of disturbance driving patch regeneration differed between riverine and reservoir sites (seasonal flooding vs. falling lake levels due to drought), but the demographic patterns did not, indicating that reservoirs can generate patch dynamics similar to those on rivers. Managing stream flows and reservoir levels to maintain disturbance cycles sufficient to generate riparian patches at different stages of regeneration through time would benefit succession-dependent species like the endangered flycatcher we studied, whether those disturbances arise from natural flooding events along free-flowing rivers or through changes in reservoir levels.